A treatment object (material) to be treated in treatment equipment such as pulverizer or tube reactor may contain an acid such as hydrochloric acid or sulfuric acid or may secondarily form such an acid during a treatment. A treatment chamber of the treatment equipment to house the treatment object is generally shaped from a steel. The acid, if present, may disadvantageously corrode the inner wall of the treatment chamber. In addition, the treatment object may often disadvantageously abrade the treatment chamber inner wall during the treatment. To prevent the disadvantages, build-up welding materials for use in machinery parts such as treatment chambers require corrosion resistance and abrasion resistance.
Fe alloys and Co alloys having chemical compositions as given in Table 1 have been used as build-up welding materials. Different build-up welding materials are used depending on the types of the treatment equipment and the treatment object.
TABLE 1Alloy chemical composition (in mass percent)CSiMnCrMoNiVWCoFeRemarksNo. 10.70.91.212.03.80.72.00.9—Bal.ConventionalExampleNo. 20.40.20.713.50.3————Bal.ConventionalExampleNo. 32.5——33.0———13.0 Bal.—ConventionalExampleNo. 41.0——26.0———5.0Bal.—ConventionalExampleNo. 50.40.20.613.00.3————Bal.ConventionalExampleNo. 64.50.71.131.01.6————Bal.ConventionalExample
All the conventional build-up welding materials, however, fail to have sufficient performance in corrosion resistance and abrasion resistance. The build-up welding materials have inferior ductility and toughness. The build-up welding materials therefore disadvantageously cause a weld overlay layer after welding to be susceptible to cracking. As a result, the build-up welding materials have inferior welding workability and often cause the weld overlay layer after welding to suffer from micro-cracks and/or coarse cracks.
Such an imperfection in welding, if generated, requires repair at that location (the location where the imperfection is generated). Specifically, a crack, if generated in the weld overlay layer, should be filled by welding of a welding material (build-up welding material) having the same chemical composition as with the weld overlay layer. The conventional build-up welding materials are, however, unsuitable for such repair. For this reason, when another overlay welding is performed on an existing weld overlay layer, the existing weld overlay layer suffers from generation of a multiplicity of cracks and fails to be sufficiently repaired.
Independently, there have been widely used treatment chambers including a weld overlay layer and, on a surface thereof, a hard chrome coated layer (having a thickness of 10 to 200 μm) so as to provide better abrasion resistance. The weld overlay metals formed from the conventional build-up welding materials, however, have poor adhesion with a hard chrome coating and fail to be covered by the hard chrome coating stably. The weld overlay metals also have inferior ductility and toughness. The weld overlay metals therefore often cause cracks in the hard chrome coated layer and/or weld overlay layer after welding.
Under such circumstances, build-up welding materials have been proposed so as to provide better corrosion resistance and abrasion resistance (see Patent Literature (PTL) 1). PTL 1 discloses a build-up welding material for use in a member that requires excellent strength, oxidation resistance, and abrasion resistance at a high temperature of 600° C. or higher. The build-up welding material contains 0.5 to 3.0 percent by weight of C, 3.0 to 7.0 percent by weight of Si, 25 to 45 percent by weight of Cr, 0 to 10 percent by weight of Mn, and 0 to 13 percent by weight of Ni, with the remainder including Fe and inevitable impurities, in which the Cr and Si contents meet a condition as specified by Cr≥−1.6Si+37. The build-up welding material forms a weld overlay metal on a base metal, where the weld overlay metal is strengthened or reinforced by fine acicular carbides in the form of short fibers.
The build-up welding material disclosed in PTL 1 employs a steel (Fe alloy). The build-up welding material contains C, Si, and Cu in high contents and contains transition elements of Groups IVa to Va (Ti, V, Zr, Nb, Mo, and Ta) in large amounts. The build-up welding material exhibits poor fracture toughness because of having a high C content of 0.5 to 3.0 percent by weight and has an excessively high hardness because of containing the transition elements of Groups IVa to Va (Ti, Zr, Nb, and Ta) in large amounts. The build-up welding material is therefore susceptible to brittle fracture by corrosion.
The build-up welding material disclosed in PTL 1 is also susceptible to red scaling (red-scale defects) after hot rolling because of having an extremely high Si content of 3.0 to 7.0 percent by weight, where the hot rolling is performed as a steel production process. The red scaling, even if removed by acid wash, may leave large surface asperities to cause surface unevenness of the product. The red scaling may possibly occur even at a high temperature of 600° C. or higher. In this case, powder of α-Fe2O3 mainly forming the red scaling migrates into the treatment object.
In addition, the build-up welding material disclosed in PTL 1 is susceptible to hot shortness (red shortness) during hot rolling process because of having an extremely high Cu content of 7.0 percent by weight in terms of its upper limit, where the hot rolling process is performed as a steel production process. This impedes the manufacturing of the steel by a common steel manufacturing method including the hot rolling process and impedes the manufacturing and supply of the target build-up welding material.
Independently, other Fe alloys have been proposed as build-up welding materials so as to provide better corrosion resistance and abrasion resistance. Specifically, there has been proposed a build-up welding material that contains 0.2 to 1.5 percent by mass of C, 0.5 to 2.0 percent by mass of Si, 0.5 to 2.0 percent by mass of Mn, 20 to 40 percent by mass of Cr, 2.0 to 6.0 percent by mass of Mo, 0.5 to 6.0 percent by mass of Ni, 1.0 to 5.0 percent by mass of V, and 0.5 to 5.0 percent by mass of W, with the remainder including Fe and inevitable impurities. The weld overlay metal includes a ferrite phase matrix and has a polycrystalline microstructure in which cementite particles are present at grain boundaries of grains of the ferrite matrix and surround the ferrite grains. The build-up welding material excels in corrosion resistance and abrasion resistance, but has room for improvements in ductility and toughness. The build-up welding material also has room for improvement in welding workability and weld repairability. Particularly, the build-up welding material, when used for another weld overlaying to repair an existing weld overlay layer, may cause cracking in the existing weld overlay layer in some weld overlaying range and may require a complicated repairing procedure. In addition, the build-up welding material has insufficient adhesion with a hard chrome coating, and this may impede the stable formation of the coating.